I would say that's plenty of power to get it over 30 fps. I have an i7 rig but with one OC gtx 275 core 731 shadders 1630 and mem 2540 and I can get around 20ish fps on those settings so your crossfire setup i bet could at average at closer to 40+
Worst case scenario: turn down the AA to 8x. Not a great deal of difference in image quality especially when you're not standing still and staring at leaves the whole time. In fact I only run 4x MSAA for that very reason unless I have a ridiculous amount of performance to "spare" like in Left 4 Dead.